1. Field of the Invention
The present invention relates to an optical part holding member made of ceramic and a production method of the optical part holding member.
2. Description of the Related Art
A cellular phone with a camera has become widespread and a camera is increasingly installed in an automobile for backward confirmation. For a photographing lens unit constituting a camera that is installed in a camera-equipped cellular phone and a camera-equipped automobile, a fairly high degree of reliability is required. In particular, since automobiles are entirely used outdoors and cellular phones are often used outdoors, it is necessary to provide performance stability and durability under severe environmental conditions such as a high temperature, a low temperature and a temperature shock caused by an abrupt change in temperature.
Accordingly, a lens holding member made of ceramic that is least affected by change in temperature has received attention.
Here, as a holding member to hold an optical part such as lens, there is reported a case in which ceramic of Sposhmen or Cordierite that possess low thermal expansion, rigidity and abrasion resistance and exhibits black color is used (Japanese Patent Application Publication No. 2002-220277). Also, for example, a proposition is made about ceramic for holding an optical part, which exhibits black color due to carbon content with a Cordierite base (Japanese Patent Application Publication No. H11-343168).
In addition, although not an application to an optical part holding member, there is a report on suppression of uneven coloring or the like at the time of sintering through blackening closely packed ceramic.
However, in the case of closely packed ceramic, the accuracy of a sintered body varies due to large shrinkage at the time of sintering, which results in the necessity to carry out a secondary processing, and poses a problem of a cost increase and the like when, for example, this is used as an optical part holding member requiring a high uniformity and a high accuracy.
Still more, there is proposed a technique capable of providing a holding member made of ceramic at a low cost in practical use by combining a precision molding technique that uses a porous ceramic part that exhibits black color without processing after the porous ceramic part is sintered (Japanese Patent Application Publication No. 2007-238430).
In addition, prevention of dew condensation is another important issue for an outdoor use.
Japanese Patent Application Publications No. 2006-19894 and No. 2006-350187 disclose that it is effective to use a permeable material for a part constituting a lens unit in a camera device to prevent dew condensation. However, complicated steps and special materials such as making a hole and disposing a permeable material in the hole are required.
Ceramic that changes little in its dimensions for a temperature change is effective for achieving performance stability for an environmental change. However, it is difficult to perform an additional process and a part incorporation for permeability of airflow similar to the prior arts, since the ceramic has high strength while being fragile.
Therefore, forming the whole entirely with a permeable material is an effective means since it eliminates after-processing. However, it is not possible to form a part as a whole entirely with a conventional material.
Further, any of the patent literatures does not describe a pore produced in a process of degreasing and sintering ceramic and the permeability by the pore, and also does not discuss an effectiveness for dew condensation.